In several branches of industry, above all in the automotive industry, metal moldings are being increasingly replaced by plastic moldings. Plastic moldings are used on the one hand to save waste and on the other hand to simplify production cycles. In the automotive industry, for example, tailgates of automobiles, wing sections and, in particular, fenders of plastic moldings are being used to increasing extent. These plastic moldings may consist of a number of different plastics or composites, including for example polyethylene, polypropylene, polycarbonate, acrylonitrile/butadiene/styrene terpolymers (ABS), polyamide, polyurethane, polyepoxides and polyesters (particularly in the form of sheet molding compounds (SMC) or bulk molding compounds (BMC)). In addition, so-called polymer blends, for example EPDM-modified polyethylenes and polypropylene materials, are often used.
These plastic moldings can be damaged in the operation of the machines, apparatus and, in particular, vehicles (automobiles) equipped with them. In particular, the fenders of automobiles are often damaged in wrecks.
Hitherto, the damaged moldings have generally been replaced by new parts and not repaired.
This approach has two major disadvantages so far as the consumer is concerned. On the one hand, it is far more expensive to replace the damaged plastic molding by a new part than to repair it by simple methods. On the other hand, the damaged plastic molding has to be disposed of. This hitherto necessary disposal of bulky and unwieldy parts is highly undesirable on environmental grounds, in addition to which the disposal costs contribute significantly towards increasing the repair costs.
Accordingly, there has been no shortage of attempts to develop processes for repairing damaged plastic moldings although they have all been found in practice to show serious disadvantages.
EP-A-186081 proposes using metal plates coated with hotmelt adhesives based on thermoplastic copolymers for bodywork repairs. The plates thus coated are then bonded over the damaged area. However, this process can only be used for metal parts and not for plastic moldings because, in this case, the area to be repaired has to be preheated to temperatures at which the hotmelt adhesive based on thermoplastic block copolymers softens, i.e. it has to be heated to temperatures above 100.degree. C. Many plastic moldings, particularly those based on thermoplastic materials, lose their dimensional stability and their mechanical properties at temperatures as high as these, so that the process proposed in EP-A-186081 is not suitable for repairing plastic moldings.
EP-A-315101 describes a method for repairing damaged articles of composite materials. According to this document, the damaged area of the composite material is mended by bonding metal or pieces of cured composite material to the surface surrounding the damaged area. The adhesive used is a rubber-modified vinyl ester resin composition. In another embodiment, an uncured reinforcing material is impregnated with a rubber-modified vinyl ester resin composition, placed over the prepared damaged zone and cured there. Graphite fiber cloths and glass fiber cloths are proposed as the reinforcing material. According to EP-A-315101, the metal or composite material is applied to the entire damaged area. There is nothing to indicate whether this repair process is also suitable for repairing plastic moldings of thermoplastic materials.
The damaged areas of plastic moldings, particularly fenders of automobiles, very often develop relatively long cracks which drastically reduce the mechanical strength of the molding. Efforts to repair such damage in the conventional way by applying reinforcing metal parts or glass fiber cloths to the entire damaged area with an adhesive have shown that the mechanical properties of the molding thus repaired are considerably poorer than those of the original undamaged molding. Where metal patches are applied to the entire damaged area, it has been found, for example in the pendulum impact test, that the bond splits over a large area. In repairs carried out with glass fiber mats, the adhesive bond begins to tear and distort the molding at the original crack in tensile tests. In pendulum impact tests, too, the fiber cloth tears at the original crack. Accordingly, there is a need to develop a process which would enable damaged plastic moldings, particularly those used in the automotive industry, to be repaired with simple means. In addition, the process would ensure that not only would the repaired part be satisfactory in appearance, it would also be equivalent to the new part in its mechanical properties.